Transmit power adjustment based on estimated electrical length of a loop

ABSTRACT

A method for estimating electrical length of a loop in a digital subscriber line (DSL) system begins by estimating an electrical length of a loop for each of a plurality signals based on a known power level of the plurality of signals, a known frequency for each of the plurality of signals, and a received power level for each of the plurality of signals. The method continues by processing the plurality of estimated electrical lengths in accordance with a function corresponding to characteristics of the loop to produce a determined electrical length.

This patent application is claiming priority under 35 USC § 120 as acontinuing patent application of patent application entitled ADJUSTMENTOF TRANSMIT POWER BASED ON AN ESTIMATED ELECTRICAL LENGTH OF A LOOP,having a filing date of Jul. 23, 2002, now U.S. Pat. No. 7,072,391 and aSer. No. 10/201,129.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention relates generally to communication systems and moreparticularly to Digital Subscriber Line (DSL) based communicationsystems.

2. Description of Related Art

Communication systems are known to enable a plurality of communicationdevices to communicate among themselves and with communication devicesin other communication systems. Such communication devices, which may becomputers, modems, facsimile machines, printers, personal digitalassistants, et cetera, communicate voice, text, and/or video data. Suchcommunication systems support the communication of data in accordancewith one or more communication standards. As is known, there are a largenumber of communication standards for the communication of data and varyfrom country to country, including a plurality of standards governingdigital subscriber line (DSL) communications. For example, the UnitedStates, Europe, Japan, China and other countries each have standards forvarious types of DSL based communications, including, but not limitedto, asynchronous digital subscriber lines (ADSL) and very high bit ratedigital subscriber line (VDSL).

As is also known, for a communication device at a customer premises toparticipate in a DSL based communication, the communication deviceincludes a DSL modem and communicates with a DSL modem at a centraloffice. The DSL modem at the customer premises is coupled to the DSLmodem at the central office via a DSL link (or loop) that typically iscomprised of an unshielded pair of wires within a multiple pair cable(i.e., a bundle of pairs of wires).

Due to the usage of multiple pair cables, the length of the DSL loop islimited by mutual interferences between the pairs of wires within thesame cable. This interference is generally known as cross-talk, whichcauses errors in the received signal and thus reduces performance of theDSL modem. Such cross-talk is either near end cross-talk (NEXT) or farend cross-talk (FEXT).

As is known, far end cross-talk of a loop in a multiple pair cable isproportional to the length of a loop and the loop transfer function. Asthe loop length increases (e.g., greater than 200 meters), theattenuation of the loop, which is an inverted logarithm of the looptransfer function, increases exponentially with respect to the length.At and above such lengths, the attenuation of the loop becomes thedominant factor and attenuates the far end cross-talk.

In a typical access network, DSL links originate at the Central Office(CO) and terminate at the Customer Premises (CPE), which are located atdifferent distances from the Central Office. As such, CPEs coupled tothe CO via shorter length loops generate significant far end cross-talk,which reduces signal-to-noise ratio (SNR) at DSL modems of the centraloffice serving longer length DSL links. The reduced SNR correspondinglyreduces upstream (i.e., from the customer premises to the centraloffice) performance of these modems, i.e., forces a lower bit rate,increases error rate, etc.

One known method to reduce far end cross-talk generated by shorter loopsis to reduce the transmit power, or power spectral density, for upstreamtransmissions based on the length and attenuation of the particularloop. To do this, the length of the loop must be determined, which canbe done by transmitting a single known signal from the DSL modem at thecentral office to the DSL modem at the customer premises. The customerpremises modem determines the attenuation of the loop based on the powerlevel of the received signal and the known power level of thetransmitted signal. The customer premises modem then determines the looplength by estimating its electrical loop length obtained by dividing theattenuation of the loop by a reference attenuation value. The referenceattenuation value may be obtained by taking the square root of thefrequency of the known signal or by some other function relating to thefrequency (f) of the known signal; for example: the referenceattenuation value may be equal to α+√f, or α+√f+β×f, where α and β arecoefficients. Alternatively, multiple signals could be transmittedyielding multiple estimated lengths, which are then averaged to achievethe final estimated electrical length.

In either of these methods for estimating the electrical loop length,errors result in an over estimation of the loop length. Such errorsoccur because the loop length estimation method does not take intoaccount inaccurate terminations at the end of the wires, mixed wiregauges, bridge taps (unloaded wire drops from the loop for anothercustomer premises) water penetration, improper splicing, et cetera. Whenthe loop length is over estimated, the transmit power is notsufficiently reduced, thus the far end cross-talk is too large andcontinues to adversely affect the performance of the other DSL modems ofthe CO in the multi-pair cable.

Therefore, a need exists for a method and apparatus that accuratelyestimates the electrical loop length and for applications thereof toreduce transmit power of DSL modems.

BRIEF SUMMARY OF THE INVENTION

The adjustment of transmit power based on an estimated electrical lengthof a loop as disclosed in the present invention substantially meetsthese needs and others. In one embodiment, a 1^(st) DSL modem at a1^(st) location (e.g., a DSL modem within the central office) transmitsa plurality of signals to a 2^(nd) DSL modem at a 2^(nd) location (e.g.,a DSL modem at the customer premises). Each of the signals transmittedby the 1^(st) DSL modem is of a known frequency and is transmitted at aknown power level (e.g., at, above, or below, the nominal transmit powerlevel of the DSL modems within the CO). Upon receiving the signals, the2^(nd) DSL modem determines the received power level for each of thesignals. The 2^(nd) DSL modem then estimates an electrical length of aloop between the 1^(st) and 2^(nd) DSL modems for each of the signalsreceived. The estimation is determined by dividing an attenuation factorby a reference attenuation value, where the attenuation factor of theloop is determined based on the known power level of the transmittedsignal and the received power level of that signal. The referenceattenuation value may be obtained by taking the square root of thefrequency of the known signal or by some other function relating to thefrequency (f) of the known signal; for example: the referenceattenuation value may be equal to α+√f, or α+√+β×f, where α and β arecoefficients.

The 2^(nd) DSL modem then processes a plurality of estimated electricallengths in accordance with a function to produce a determined electricallength. The function corresponds to the characteristics of a loopbetween the 1^(st) and 2^(nd) DSL modems (e.g., frequency response,attenuation, gain response, et cetera). In an embodiment of the presentinvention, the function selects the estimated electrical length havingthe smallest value as the determined electrical length. Havingdetermined the electrical length, the 2^(nd) DSL modem adjusts itstransmit power accordingly, which reduces far-end cross talk.

By utilizing the function to determine the electrical length of theloop, a more accurate estimate of the electrical length is obtained. Byobtaining a more accurate estimate of the electrical length, thetransmit power may be reduced to a more appropriate level, thus reducingthe far end cross-talk. By reducing the far end cross-talk, DSLcommunications occurring within the multi-pair cable that includes thisparticular DSL loop is improved.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating a DSL system inaccordance with the present invention;

FIG. 2 is a schematic block diagram of two DSL modems communicating inaccordance with the present invention;

FIGS. 3-5 are graphical representations of examples of variousattenuation responses of DSL loops of the DSL system of FIG. 1;

FIG. 6 is a graphical representation of a DSL channel frequency spectrumin accordance with the present invention;

FIG. 7 is a graphical representation of a plurality of signals inaccordance with the present invention;

FIG. 8 is a graphical representation of a plurality of received signalsin accordance with the present invention;

FIG. 9 is a graphical representation of a plurality of attenuations fora DSL loop in accordance with the present invention;

FIG. 10 is a graphical representation of a plurality of estimatedelectrical lengths of a DSL loop in accordance with the presentinvention;

FIG. 11 is a logic diagram of a method for adjusting transmit powerbased on an estimated electrical length of a loop in accordance with thepresent invention; and

FIG. 12 is a logic diagram of an alternate method for adjusting transmitpower based on an estimated electrical length of a loop in accordancewith the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a DSL system 10 that includes a central office 12 anda plurality of customer premises 14-18. The central office 12 includes aplurality of DSL. modems 20-24. Each of the customer premises includes aDSL modem 26-30. Each DSL modem 26-30 at a customer premises 14-18 iscoupled via a twisted pair of wires with a corresponding DSL modem 20-24in the central office. The twisted pairs between the central office 12and the plurality of customer premises 14-18 are bundled 32 to produce amulti-pair cable. As is known, a byproduct of the bundling 32 of thetwisted wire pairs results in cross-talk interference between thetwisted pairs that reduces the performance of the overall DSL system 10.

As one of average skill in the art will appreciate, the DSL system 10may include multiple central offices and many more customer premisesthan shown in FIG. 1. For DSL systems that include multiple centraloffices, the central offices are tied together through a communicationsystem component such as a regional branch exchange.

FIG. 2 illustrates a 1^(st) DSL modem communicating to a 2^(nd) DSLmodem. The 1^(st) DSL modem typically corresponds to one of the DSLmodems 26-30 within the central office (CO) 12 of FIG. 1 and the 2^(nd)DSL modem may correspond to one of the DSL modems 20-24 at the customerpremises (CPE) 14-18. While this is the typical configuration, theplurality of signals 54 may be transmitted from the DSL modem at the CPEto a DSL modem within the CO for determining transmit power levels forthe DSL modems in the CO and/or the DSL modem at the CPE. The remainderof the discussion will be based on the typical configuration.

As shown, DSL modem #1 includes a transceiving module 40 and an optionalpower adjust module 42. The 2^(nd) DSL module is shown to include atransceiving module 44, a determining module 46, an estimating module48, a processing module 50 and an adjusting module 52. As one of averageskill in the art will appreciate, the transceiver modules 40 and 44include a transmitter, receiver and a hybrid circuit that converts a2-wire twisted pair into a 4-wire connection. Accordingly, 2 of the 4wires are used for the transmitter and the other 2 of the 4 wires areused for the receiver. As such, signals transmitted between the DSLmodems are communicated via the transceiving modules 40 and 44.

The transceiving module 40 and the optional adjusting module 42 of the1^(st) DSL modem and the transceiving module 44, determining module 46,estimated module 48, processing module 52 and the adjusting module 52 ofthe 2^(nd) DSL modem may be implemented as a single device in each DSLmodem or a plurality of devices. Such a device may be a singleprocessing device or a plurality of processing devices and may furtherinclude memory. Such a processing device may be a microprocessor,microcontroller, digital signal processor, microcomputer, centralprocessing unit, field programmable gate array, programmable logicdevice, state machine, logic circuitry, analog circuitry, digitalcircuitry, and/or any device that manipulates signals (analog and/ordigital) based on operational instructions. The memory may be a singlememory device or a plurality of memory devices. Such a memory device maybe a read-only memory, random access memory, volatile memory,non-volatile memory, static memory, dynamic memory, flash memory, and/orany device that stores digital information. Note that when a deviceimplements one or more of its functions via a state machine, analogcircuitry, digital circuitry, and/or logic circuitry, the correspondingoperational instructions are embedded with the circuitry comprising thestate machine, analog circuitry, digital circuitry, and/or logiccircuitry. In general, the memory stores, and the processing deviceexecutes, operational instructions corresponding to at least some of thesteps and/or functions illustrated in FIGS. 2-12.

In operation, the 1^(st) DSL modem transmits a plurality of signals 54to the 2^(nd) DSL modem. Each of the plurality of signals is of a knownfrequency and is transmitted at a known power level. The frequency rangefor the plurality of signals may correspond to a DSL channel frequencyspectrum or a portion thereof. As an example for a VDSL system, theplurality of signals 54 may be contained in the down stream bands of aDSL channel and span from a few kilohertz to 12 megahertz with a spacingof 100-500 kilohertz.

The transceiving module 44 provides the plurality of signals 54 to thedetermining module 46. The determining module 46, which may be areceived signal strength indicator, signal-to-noise ratio module, and/orsignal-to-interference module, determines the received power level 56for each of the plurality of signals 54.

The determining module 46 provides the received power level 56 for eachof the signals 54 to the estimating module 48. The estimating module 48estimates an electrical length 58 for each of the signals 54 from thecorresponding received power level 56. In one embodiment, the estimatingmodule 48 determines the electrical length 58 of a transmitted signal 54by determining an attenuation factor based on the known power level ofthe transmitted signal and the received power level of the transmittedsignal and then dividing the attenuation factor by a referenceattenuation value. The reference attenuation value may be obtained bytaking the square root of the frequency of the known signal or by someother function relating to the frequency (f) of the known signal; forexample: the reference attenuation value may be equal to α+√f, orα+√f+β×f, where α and β are coefficients. Once the estimating module 48has calculated the estimated length for each of the plurality ofsignals, it provides a plurality of estimated electrical lengths 58 tothe processing module 50.

The processing module 50, based on a function 64 that corresponds to thecharacteristics of the loop between the 1.sup.st and 2.sup.nd DSLmodems, determines an electrical length 60 from the plurality ofestimated lengths 58. The characteristics of the loop may beattenuation, frequency response, gain response, impedance, and/or anyother electrical and/or electromagnetic properties of a transmissionwire. In one embodiment, the processing module 50, based on the function64, determines the estimated electrical length 60 from the plurality ofestimated electrical lengths 58 as the one having the shortest length.In an alternative embodiment, the processing module 50 provides aweighting factor for each of the estimated electrical lengths 58. Theweighting factor is based on the inverse of the estimated electricallength. As such, estimated electrical lengths that are shorter are givena much greater weighting factor than estimated electrical lengths ofgreater values. Having weighted each of the estimated electricallengths, the processing module 50 computes the determined electricallength as an average, root mean square and/or least mean square of theplurality of weighted estimated electrical lengths.

The processing module 50 provides the determined electrical length 60 tothe adjusting module 52. Based on the determined electrical length 60,the adjusting module 52 generates a transmit power adjust signal that isprovided to the transceiving module 44, which adjusts it transmit poweraccordingly. The transmit power adjust signal and corresponding theamount of reduction of transmit power may be done in accordance with oneor more of the standards governing DSL transmissions. Such standardsinclude, but are not limited to ITU-T, G.993.1, TI-424, and ETSITS-101-270-2.

If the first DSL modem includes the optional power adjust module 42,processing module 50 provides signal strength information 61 to thetransceiving module 44, where the signal strength information 61 may bethe plurality of received power levels 56, the plurality of estimatedelectrical lengths 58, and/or the determined electrical length. Thetransceiving module 44 transmits the signal strength information 61 tothe transceiving module 40 of DSL modem #1. The transceiving module 40provides the determined signal strength information 61 to the poweradjust module 42. Based on the signal strength information 61 from DSLmodem #2 and/or from other DSL modems at various other CPEs, the poweradjust module 42 generates a transmit power adjust signal 41 that isprovided to the transceiving module 40, which adjusts its transmit powerin accordance with the transmit power adjust signal. As one of averageskill in the art will appreciate, the DSL modems within the CO all usethe same nominal transmit power. As such, if the transmit power of theDSL modems within the CO, it will be adjusted based on loop length ofmultiple DSL modems at the CPEs.

FIG. 3 illustrates a frequency versus attenuation plot of a homogeneousloop (i.e., a loop of the same wire gauge throughout with no bridgedtaps and proper termination of the ends). The illustration includes 2loops of various lengths. Length 1 is significantly longer than length 2and thus has a greater attenuation. For both loops, the attenuationincreases nearly exponentially as the frequency increases. When the loopbetween the 1^(st) and 2^(nd) DSL modem, as shown in FIG. 2 is known tobe a homogeneous loop, as shown in FIG. 3, the number of signalsgenerated by the 1^(st) DSL modem may be a relatively small number (1-4)since the loop's performance is predictable as shown in FIG. 3.

FIG. 4 illustrates examples of attenuation versus frequency graph for 2non-homogeneous loops of different lengths, where a non-homogeneous loopincludes improper terminations, different wire gauges, impropersplicing, et cetera. As shown, the attenuation increases as thefrequency increases in a non-uniform exponential manner. If the loopbetween the 1^(st) and 2^(nd) DSL modems of FIG. 2 is known to be anon-homogeneous loop as illustrated in FIG. 4, the entire down streambands of the DSL channel does not need to be swept (i.e., the pluralityof signals does not need to span the entire frequency spectrum of thedown stream bands of the DSL channel). However, since the performance ofthe non-homogeneous loop is not as predictable as the homogeneous loopof FIG. 3, more signals are required to obtain an accuraterepresentation of the electrical length of the loop.

FIG. 5 illustrates a graph of attenuation versus frequency for 2non-homogeneous loops of various lengths that include bridged taps. Asshown, the attenuation dramatically varies from a homogeneous loop. Assuch, if the loop between the 1^(st) and 2^(nd) DSL modems of FIG. 2includes bridged taps as shown in FIG. 5, the entire spectrum of thedown stream bands should be swept (i.e., a signal should be generatedthat corresponds to the entire frequency spectrum of the down streambands of the DSL channel) to obtain an accurate representation of theloop.

FIG. 6 illustrates a graphical representation of a DSL channel frequencyspectrum that includes two down stream frequency bands and one up streamfrequency band. In this illustration, the channel spectrum ranges from alow frequency (f_(L)) to a high frequency (f_(H)). The range offrequencies between f_(L) and f_(H) is dependent on the particularstandard being implemented. For example, for a VDSL, the frequency rangemay be from a few kilohertz to 12 megahertz.

FIG. 7 illustrates a frequency plot of the plurality of signals at thetransmit side (e.g., transmitted by DSL modem #1 of FIG. 2) that spansthe entire frequency spectrum of the down stream bands of the DSLchannel. As mentioned with reference to FIGS. 3 and 4, the plurality ofsignals is not always required to span the entire channel frequencyspectrum of the down stream bands and accordingly may span only aportion of one or both bands. However, if the characteristics of theloop are unknown and/or the loop is known to include bridged taps, thepreferred embodiment is to sweep a majority, if not all, of the channelfrequency spectrum of the down stream bands illustrated in FIG. 6.

FIG. 8 illustrates a plot of the signals as received by the 2^(nd) DSLmodem. As shown, if the loop were a homogeneous loop, each of thecorresponding receive signals would have a power level corresponding tothe homogeneous curve of the receive power levels. Note that thehomogeneous curve is based on the power levels of the signalstransmitted in FIG. 7 being of equal value. As one of average skill inthe art will appreciate, the signals of FIG. 7 in the higher frequencyranges may have power levels that are different from nominal powerlevels to compensate for the attenuation of the loop such that thereceived signal power level is more uniformed.

FIG. 9 illustrates a plot of the plurality of attenuations that aregenerated from the plurality of signals in FIG. 8. As shown, however,several received signals have substantially greater attenuation than thehomogeneous curve while others have only slightly greater attenuationthan the homogeneous curve. If only one of the these signals were usedto estimated the electrical length, as in one prior art method, theelectrical length may be dramatically over estimated since theattenuation for a particular signal may be substantially greater thanthe attenuation of the entire loop.

FIG. 10 illustrates a plurality of estimated electrical lengths that maybe derived from the plurality of attenuations. The estimated electricallengths may be determined by dividing the attenuation of a correspondingsignal in FIG. 9 by a reference attenuation value. The referenceattenuation value may be obtained by taking the square root of thefrequency of the known signal or by some other function relating to thefrequency (f) of the known signal; for example: the referenceattenuation value may be equal to α+√f, or α+√f+β×f, where α and β arecoefficients. As shown, if the loop were homogeneous, each of theestimated electrical lengths would have about the same value, whichcorresponds to the homogeneous estimation of length. As shown, theactual estimated electrical lengths for this example have several valuesjust above the homogeneous electrical length and other significantlyabove the homogeneous loop length. In one embodiment, the lowest actualestimated electrical length may be used as the electrical length for theloop. In an alternate embodiment, the plurality of estimated electricallengths may be given a weighting factor that is proportional to theinverse of the magnitude of the estimated electrical length. As such,estimated lengths closer to the homogeneous estimation are given muchgreater weight than those farther away from the homogeneous estimation.In this manner, anomalies within the loop may be more accuratelyaccounted for.

As one of average skill in the art will appreciate, if the properties ofthe loop are known, only a portion of the plurality of signals needs tobe generated. For example, if the loop is known to include bridged tapsas shown in FIG. 5, the plurality of signals may be generated to includefrequencies that encompass a minimal attenuation value for the loop,i.e., from one peak to the next. Similarly, the plurality of signals mayinclude a smaller number when the loop is known to have the propertiesillustrated in FIG. 3 or 4.

FIG. 11 illustrates a logic diagram of a method for adjusting transmitpower of a DSL modem in a DSL system based on an estimated electricallength of a loop. The process begins at Step 70 where a 1^(st) DSL modemat a 1^(st) location transmits a plurality of signals to a 2^(nd) DSLmodem at a 2^(nd) location. The 1^(st) location may be at the centraloffice and the 2^(nd) location may be at the customer premises.Typically, the plurality of signals will be transmitted during atraining mode for the loop and/or during pilot signaling. Each of thesignals transmitted by the 1^(st) DSL modem have a known frequency andis transmitted at a known power level. The frequencies of the signalsmay span the entire frequency spectrum of the down stream bands of a DSLchannel, or all channels in the DSL system, or a portion thereof. If thetype of loop between the 1^(st) and 2^(nd) DSL modems is known, thegeneration of the plurality of signals may be tailored to the particulartype of loop.

The process then proceeds to Step 72 where the 2^(nd) DSL modemdetermines received power levels for each of the plurality of signals itreceives. This may be done by utilizing a received signal strengthindication, signal-to-noise ratio, signal-to-interference ratio, and/orany other means for determining the signal strength of a receivedsignal.

The process then proceeds to Step 74 where the 2^(nd) DSL modemestimates an electrical length of the loop for each signal it receives.The estimation may be done by determining an attenuation factor based onthe known power level that the signal was transmitted at and thereceived power level of the signal. The processing continues by dividingthe attenuation factor by the reference attenuation value. The referenceattenuation value may be obtained by taking the square root of thefrequency of the known signal or by some other function relating to thefrequency (f) of the known signal; for example: the referenceattenuation value may be equal to α+√f, or α+√f+β×f, where α and β arecoefficients.

The 2^(nd) DSL modem then processes the plurality of estimatedelectrical lengths in accordance with a function at Step 76. Thefunction corresponds to characteristics of the loop between the 1^(st)and 2^(nd) DSL modems. The function may be selecting one of theplurality of estimated electrical lengths that has the smallest value.Alternatively, each of the plurality of estimated electrical lengths maybe assigned a weighting factor that is non-linear and proportional tothe inverse of the magnitude of the estimated length. The processingthen would continue by computing the determined electrical length byaveraging, producing a root mean square and/or producing a least meansquare of the plurality of weighted estimated electrical lengths. Theprocess then proceeds to Step 78 where the 2^(nd) DSL modem adjusts itstransmit power based on the determined electrical length.

FIG. 12 illustrates an alternate method for adjusting transmit power ina DSL system based on an estimated electrical length of a loop. Theprocess begins at Step 90 where an electrical length of a loop isestimated for each of a plurality of signals based on a known powerlevel of the signals when transmitted, a known frequency for each of theplurality of signals and a received power level for each of theplurality of signals. The process then proceeds to Step 92 where theplurality of estimated electrical lengths are processed in accordancewith a function to produce a determined electrical length. The processthen proceeds to Step 94 where the transmit power of a DSL modem coupledto the loop is adjusted based on the determined electrical length.

The preceding discussion has presented a method and apparatus foradjusting transmit power of DSL modems in a DSL system based on anestimated electrical length of a loop. By more accurately determiningthe estimated electrical length, the power adjustment is more accuratethus reducing far end cross-talk. By reducing far end cross-talk in aDSL system, the overall system performs better. As one of average skillin the art will appreciate, other embodiments may be derived from theteaching of the present invention, without deviating from the scope ofthe claims.

1. A method for estimating electrical length of a loop in a digitalsubscriber line (DSL) system, the method comprises: estimating anelectrical length of the loop for each of a plurality of signals basedon a known power level of the plurality of signals, a known frequencyfor each of the plurality of signals, and a received power level foreach of the plurality of signals; and selecting one of the plurality ofestimated electrical lengths having a smallest value to produce adetermined electrical length of the loop.
 2. The method of claim 1,wherein the estimating the electrical length of the loop for each of theplurality of signals comprises: receiving the plurality of signals,wherein each signal of the plurality of signals is transmitted at theknown power level; and determining the received power level for eachsignal of the plurality of signals as the signal is received.
 3. Themethod of claim 1, wherein the estimating the electrical length of theloop for a signal of the plurality of signals comprises: determining anattenuation factor based on the known power level and the received powerlevel of the signal; and dividing the attenuation factor by a referenceattenuation value to produce a corresponding one of the plurality ofestimated electrical lengths, wherein the reference attenuation value isobtained by taking a square root of the frequency of the signal, bytaking the square root of the frequency of the signal and adding a firstcoefficient thereto, or by taking the square root of the frequency ofthe signal and adding a first coefficient thereto and adding a productof the frequency of the signal and a second coefficient.
 4. An apparatusfor estimating electrical length of a loop in a digital subscriber line(DSL) system, the apparatus comprises: means for estimating anelectrical length of the loop for each of a plurality of signals basedon a known power level of the plurality of signals, a known frequencyfor each of the plurality of signals, and a received power level foreach of the plurality of signals; and means for selecting one of theplurality of estimated electrical lengths having a smallest value toproduce a determined electrical length of the loop.
 5. The apparatus ofclaim 4, wherein the means for estimating the electrical length of theloop for each of the plurality of signals functions to: receive theplurality of signals, wherein each signal of the plurality of signals istransmitted at the known power level; and determine the received powerlevel for each signal of the plurality of signals as the signal isreceived.
 6. The apparatus of claim 4, wherein the means for estimatingthe electrical length of the loop for a signal of the plurality ofsignals functions to: determine an attenuation factor based on the knownpower level and the received power level of the signal; perform a squareroot function of the known frequency for the signal to produce afrequency value; and divide the attenuation factor by the frequencyvalue to produce a corresponding one of the plurality of estimatedelectrical lengths.
 7. A method for estimating electrical length of aloop in a digital subscriber line (DSL) system, the method comprises:estimating an electrical length of the loop for each of a plurality ofsignals based on a known power level of the plurality of signals, aknown frequency for each of the plurality of signals, and a receivedpower level for each of the plurality of signals; weighting each of theplurality of estimated electrical lengths to produce a plurality ofweighted estimated electrical lengths; and computing a determinedelectrical length of the loop as a function of the plurality of weightedestimated electrical lengths.
 8. The method of claim 7, wherein theestimating the electrical length of the loop for each of a pluralitysignals comprises: receiving the plurality of signals, wherein eachsignal of the plurality of signals is transmitted at the known powerlevel; and determining the received power level for each signal of theplurality of signals as the signal is received.
 9. The method of claim7, wherein the estimating the electrical length of the loop for a signalof the plurality of signals comprises: determining an attenuation factorbased on the known power level and the received power level of thesignal; and dividing the attenuation factor by a reference attenuationvalue to produce a corresponding one of the plurality of estimatedelectrical lengths, wherein the reference attenuation value is obtainedby taking a square root of the frequency of the signal, by taking thesquare root of the frequency of the signal and adding a firstcoefficient thereto, or by taking the square root of the frequency ofthe signal and adding a first coefficient thereto and adding a productof the frequency of the signal and a second coefficient.
 10. The methodof claim 7, wherein the weighting each of the plurality of estimatedelectrical lengths comprises: determining for each of the plurality ofestimated electrical lengths a weighting factor that is proportional toan inverse of a magnitude of the estimated electrical length; andweighting each of the plurality of estimated electrical lengths by theirrespective weighting factor.
 11. The method of claim 7, whereincomputing the determined electrical length comprises: computing at leastone of an average, a root mean square, and a least mean square of theplurality of weighted estimated electrical lengths.
 12. The method ofclaim 11, wherein each of the plurality of estimated electrical lengthsare weighted such that smaller valued ones of the plurality of estimatedelectrical lengths have a greater weighting factor than larger valuedones of the plurality of estimated electrical lengths.
 13. An apparatusfor estimating electrical length of a loop in a digital subscriber line(DSL) system, the apparatus comprises: means for estimating anelectrical length of the loop for each of a plurality of signals basedon a known power level of the plurality of signals, a known frequencyfor each of the plurality of signals, and a received power level foreach of the plurality of signals; and means for weighting each of theplurality of estimated electrical lengths to produce a plurality ofweighted estimated electrical lengths; and means for computing adetermined electrical length of the loop as a function of the pluralityof weighted estimated electrical lengths.
 14. The apparatus of claim 13,wherein the means for estimating the electrical length of the loop foreach of the plurality signals functions to: receive the plurality ofsignals, wherein each signal of the plurality of signals is transmittedat the known power level; and determine the received power level foreach signal of the plurality of signals as the signal is received. 15.The apparatus of claim 13, wherein the means for estimating theelectrical length of the loop for a signal of the plurality of signalsfunctions to: determine an attenuation factor based on the known powerlevel and the received power level of the signal; perform a square rootfunction of the known frequency for the signal to produce a frequencyvalue; and divide the attenuation factor by the frequency value toproduce a corresponding one of the plurality of estimated electricallengths.
 16. The apparatus of claim 13, wherein the means for weightingeach of the plurality of estimated electrical lengths functions to:determine for each of the plurality of estimated electrical lengths aweighting factor that is proportional to an inverse of a magnitude ofthe estimated electrical length; and weight each of the plurality ofestimated electrical lengths by their respective weighting factor. 17.The apparatus of claim 13, wherein the means for computing thedetermined electrical length functions to: compute at least one of anaverage, a root mean square, and a least mean square of the plurality ofweighted estimated electrical lengths.
 18. The apparatus of claim 17,wherein each of the plurality of estimated electrical lengths areweighted such that smaller valued ones of the plurality of estimatedelectrical lengths have a greater weighting factor than larger valuedones of the plurality of estimated electrical lengths.